Office 2007 is commonly sold in media-less license mode which means you have to send away to get the actual installation disk.  Or, it may have come pre installed on your PC.  If you need to get a backup copy, you can download it from the Microsoft website.  That way if you need to add a component or reinstall the software, you will have your disk handy.  To download, you will need to have your product code.

http://trial.trymicrosoftoffice.com/msft-orpc/registration.aspx#